Isil still holds several districts along the southern bank of Lake Assad and part of dam itself, but a month under siege has considerably weakened the group.

As the SDF knocked holes in Isil’s presence in Tabqa, civilians slipped through the gaps and headed to the shore of Lake Assad.

There, the rickety fishing boats and barges that haul SDF fighters and supplies into Tabqa spirit away locals who have lived through years of Isil rule.

"People are hungry and tired. Everyone is psychologically shattered, crushed," said 39-year-old Ismail Mohamed, who arrived at the water corridor with his family.

"When we got on the water, riding the boat, we truly couldn't believe it, we were so happy," he told AFP news agency.

Once the SDF controls Tabqa completely, it is expected to move south towards Raqqa.

Nearly three years into Operation Inherent Resolve, the US-led military campaign against Isil, Raqqa is no longer at the physical heart of Isil’s operations, but it remains a population stronghold and symbolically valuable asset for the group.

Meanwhile, an Iraqi commander expects to dislodge Isil from Mosul in May despite resistance from militants in the densely populated Old City district.

The battle should be completed "in a maximum of three weeks", the Iraqi army's chief of staff, Lieutenant General Othman al-Ghanmi, was quoted as saying by state-run newspaper al-Sabah on Sunday.

A US-led international coalition is providing air and ground support for the offensive.

The United Nations believes up to half a million people remain in the area controlled by the militants, 400,000 of whom are in the Old City with little food and water and no access to hospitals.

On Sunday, the United Nations said a group of 36 Yazidi survivors had been rescued after three years of "slavery" under Isil.

Since Friday, the women and girls from the group had been receiving lodging, clothing, medical and psychological aid in Duhok, a Kurdish city north of Mosul, said a statement from Lise Grande, the UN Humanitarian Coordinator for Iraq.

The Yazidis, whose beliefs combine elements from several Middle Eastern religions, were the most persecuted community under Isil, which considers them devil-worshippers.

The UN estimates that up to 1,500 Yazidi women and girls remain in captivity, suffering abuse.